1. Field of the Invention
This invention relates to the production of glass fibres, and more specifically to apparatus and methods for the production of glass fibres by the attenuation of glass streams flowing in molten form from a bushing. The invention is particularly applicable to such apparatus operating by the so-called direct melt process, in which molten glass is supplied direct from a glass batch melting and fining furnace to the bushing.
2. Description of the Prior Art
British patent specification Nos. 976,472 and 1,214,920 contain descriptions of forms of the direct melt process, and illustrate in particular how molten glass is conveyed to a multiplicity of bushings from the glass-melting furnace along a series of channels or fore-hearths. The fore-hearths are shown as being arranged in an "H" pattern with the bushings arranged along each of the branches of the "H". The bushings are in each case mounted below an opening formed in the bottom wall or floor of the fore-hearth, the opening being defined by an aperture in a specially formed refractory block, known as a "flow block". Such flow blocks and the problems encountered in feeding molten glass to a bushing are discussed in more detail in British patent specification Nos. 1,076,264 and 1,081,643. Further systems for mounting bushings below a fore-hearth are illustrated in FIG. 3 of British patent specification No. 923,602 and FIG. 3 of British patent specification No. 908,138.
The bushing itself is generally electrically heated and essentially comprises a horizontal plate provided with a plurality of downwardly projecting tips, each having a bore or orifice through which a stream of molten glass can flow. The streams are drawn out to attenuate them and form the individual filaments, which are then grouped into strands. The bushing plate may form the bottom wall of a trough which is attached to the underside of the fore-hearth.
The refractory blocks of which the fore-hearth is made have a relatively high thermal conductivity, so that the molten glass, in its passage through the fore-hearth floor opening into the bushing, is inevitably cooled to a certain extent where it is in contact with the refractory block or blocks defining the opening. A temperature gradient is thus established in the glass, with the glass adjacent to the refractory blocks being cooler than the glass flowing through the central part of the opening.
In cases where the glass being used is prone to devitrification if subjected to any substantial cooling, the cooling of the glass adjacent to the refractory blocks can result in a build-up of devitrified glass on the surface of the refractory. Small quantities of such devitrified glass would probably remain safely attached to the refractory, but as the build-up continues the chances of particles of devitrified glass being dislodged are increased, until a stage is reached at which such particles are continually being dislodged and interfere with the operation of the bushing. The frequency of breakages of the strands increases, because the particles of devitrified glass cause individual filaments to break, and once one filament has broken the effect is to cause other filaments to break until the strand has broken. The operator must then stop drawing, remove the glass fibre already drawn, and re-start the process.
This problem has been encountered particularly with a glass having a high content of zirconia, such as we have described in our British patent specification No. 1,290,528, when forming fibres for reinforcing cementitious materials. After a relatively short operating time, using the apparatus described above, a stage has been reached where the strand breakage frequency is unacceptable for large scale continuous commercial production. Further, the more stoppages are thus caused the worse the problems become, due to temperature fluctuations caused by the stoppages.